首先，先看代码内容：（但愿你们本身能够运行尝试，以加深记忆和理解）java
package s11;this
import java.util.Comparator;spa
import java.util.TreeSet;code
class Student implements Comparable {排序
String name;class
int age;import
int classNum;im
public Student() {demo
}static
public Student(String name, int age, int classNum) {
this.name = name;
this.age = age;
this.classNum = classNum;
}
public String toString() {
return "Student [name=" + name + ", age=" + age + ", classNum="
+ classNum + "]";
}
public int compareTo(Object o) {
Student s = (Student) o;
return this.classNum - s.classNum;
}
}
public class Studentdemo {
public static void main(String[] args) {
TreeSet ts1 = new TreeSet(
new Comparator() {
public int compare(Object o1,Object o2) {
Student s1 = (Student)o1;
Student s2 = (Student)o2;
return s1.age > s2.age ? 1 : -1;
}
});
ts1.add(new Student("mm",21,97005));
ts1.add(new Student("jerry",19,97003));
ts1.add(new Student("tom",16,97004));
ts1.add(new Student("mm",28,97008));
ts1.add(new Student("mm",23,97006));
System.out.println("语句return s1.age > s2.age ? 1 : -1;是按照？？？排列的：");
System.out.println(ts1);
}
}
这是运行结果：
语句return s1.age > s2.age ? 1 : -1;是按照升序（由小到大）排列的：
[Student [name=tom, age=16, classNum=97004],
Student [name=jerry, age=19, classNum=97003],
Student [name=mm, age=21, classNum=97005],
Student [name=mm, age=23, classNum=97006],
Student [name=mm, age=28, classNum=97008]]
也就是说当语句return s1.age > s2.age ? 1 : -1;的返回值为1时，也就是说 s1的值大于s2的值时 ，compareTo是按照升序（由小到大）排序的！
当返回值为-1时，也就是说 s1的值小于s2的值时 ，compareTo是按照降序（由大到小）排序的！
当返回值为0时，s1等于s2的值，若是数值所有相等则排序也一样按照初始顺序排列，若是只有前两个数值相等，那么，compareTo会继续比较下一组s1 与 s2的值，一样， s1的值大于s2的值时 ，compareTo是按照升序（由小到大）排序的， s1的值小于s2的值时 ，compareTo是按照降序（由大到小）排序的！但愿你们今天能完全明白，不枉辛苦一字，谢谢！